Agriculture Reference
In-Depth Information
6. As water drains from a saturated soil, matric suction forces acting on the
remainingwatergraduallyincrease.Asoilwaterretentioncurve,unique
to each soil, describes the change in soil water content as the matric
suction increases. At the
FC
, the matric suction is normally 10 kPa, and
this increases to 1500 kPa at the
PWP
. The water held between 10 kPa
and60kPaisconsideredtobereadilyavailablewater(
RAW
)—vines
do not suffer any stress in this range; that held between 60 kPa and 400
kPaisdeicitavailablewater(
DAW
),becauseinthisrangevinessufer
moderate water stress, with symptoms observable in the vineyard.
7. The fraction of a soil's volume that is drained at the
FC
defines its
air-illedporosity,whichshouldbeatleast10%foradequategas
exchange(aeration).Anaerobicconditionsthatareunfavorabletoroot
functionarelikelytooccurwhentheair-illedporosityis<10%.
8. Soilcompaction,whichisobviousinsodicsubsoilswhendry,causes
anexcessiveincreaseinsoilstrength.Soilstrength,measuredwitha
penetrometer, should ideally be <2 MPa when the soil is at its
FC
.Such
a condition provides a sufficiently firm base for machinery, promotes
good drainage, and allows easy root penetration.
9. Vineyardsoilsinwinterrainfallregionsshouldnormallyregain
FC
by
the end of winter. After bud burst, evaporation from the soil surface and
transpirationthroughthevines(togetherevapotranspiration,or
ET
)
createasoilwaterdeicit(
SWD
).he
SWD
increases as vines first extract
RAW
and then
DAW
. For the same depth of soil, the sum of
RAW
and
DAW
relative to
AWC
is greatest for sandy clay loams and clay loams
andleastforheavyclays.hepotentialplantavailablewater(
PAW
)is
the product of
AWC
and the soil depth.
10. In dry-grown vineyards, the supply of
RAW
plus
DAW
depends on
the balance between rainfall and
ET
, soil texture, and the depth of
rooting. In irrigated vineyards, depth of rooting is less important
because irrigation is used to control the water content of the top 50 cm
of soil only. In both cases, moderate water stress from fruit set to
veraisonenhancesfruitqualityandthelavorandaromasofthewines
produced.Inirrigatedvineyards,regulateddeicitirrigation(RDI)is
used to control
SWD
in a desired range and also saves water. Partial root
zonedrying(PRD)underirrigationcanalsobeusedtosavewaterand
enhance fruit flavors without serious yield loss.
11. Soilwatermonitoringbymeansoftensiometers,gypsumblocks,
capacitance probes, or time domain reflectometery can be used to
schedule irrigations and indicate the amount of water to be applied.
Vine water stress is assessed directly by measuring a leaf or stem water
potential.Soilwaterstatuscanalsobeestimatedfrommeasurements
